Current development of liquid-repellent surfaces is inspired by the self-cleaning abilities of many natural surfaces on animals, insects, and plants. Water droplets on these natural surfaces maintain a near-spherical shape and roll off easily, carrying dirt away with them. The water-repellency function has been attributed to the presence of micro/nanostructures on many of these natural surfaces. These observations have led to enormous interest in the past decade in manufacturing biomimetic water-repellent surfaces, owing to their broad spectrum of potential applications, which range from water-repellent fabrics to friction reduction surfaces.
More specifically, synthetic liquid-repellent surfaces in the art are inspired by the lotus effect (Barthlott, W. & Neinhuis, C. Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 202, 1-8 (1997)) in which water droplets are supported by surface textures on a composite solid/air interface that enables water droplets to easily roll off the surface (Cassie, A. B. D. & Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 40, 0546-0550 (1944); Cassie, A. B. D. & Baxter, S. Large contact angles of plant and animal surfaces. Nature 155, 21-22 (1945)). However, this approach has inherent limitations that severely restrict its applicability. First, trapped air is a largely ineffective cushion against organic fluids or complex mixtures that, unlike water, have low surface tension that strongly destabilizes suspended droplets (Shafrin, E. G. & Zisman, W. A. Constitutive relations in the wetting of low energy surfaces and the theory of the retraction method of preparing monolayers. J. Phys. Chem. 64, 519-524 (1960)).
Moreover, air trapped within surface textures cannot withstand pressure, so that liquids—particularly those with low surface tension—can easily penetrate the surface texture under even slightly elevated pressures or upon impact, conditions commonly encountered with driving rain or in underground transport pipes (Nguyen, T. P. N., Brunet, P., Coffinier, Y. & Boukherroub, R. Quantitative testing of robustness on superomniphobic surfaces by drop impact. Langmuir 26, 18369-18373 (2010)). Furthermore, synthetic textured solids are prone to irreversible defects arising from mechanical damage and fabrication imperfections (Quere, D. Wetting and roughness. Annu. Rev. Mater. Res. 38, 71-99 (2008); Bocquet, L. & Lauga, E. A smooth future? Nature Mater. 10, 334-337 (2011)). Because each defect enhances the likelihood of the droplet pinning and sticking in place, textured surfaces are not only difficult to optimize for liquid mobility but inevitably stop working over time as damage accumulates. Recent progress in pushing these limits with increasingly complex structures and chemistries remains outweighed by substantial tradeoffs in physical stability, optical properties, large-scale feasibility, and/or difficulty and expense of fabrication (Tuteja, A. et al., Science 318, 1618-1622 (2007); Tuteja, A., et al., Proc. Natl. Acad. Sci. USA 105, 18200-18205 (2008); Ahuja, A., et al., Langmuir 24, 9-14 (2008); Li, Y., et al., Angew. Chem. Int. Ed. 49, 6129-6133 (2010)).
Despite over a decade of intense research, surfaces in the art are still plagued with problems that restrict their practical applications: they exhibit limited oleophobicity with high contact angle hysteresis; fail under pressure; cannot self-heal when damaged; and are expensive to produce.
For example, no surfaces that delay or prevent blood clotting, a process that relies on adhesion of platelets and proteins to a surface as a first step, have been developed. Soluble anti-coagulants, such as heparin, must be added to flowing blood in any extracorporeal shunt to prevent clot formation. Certain polymeric species, such as polyethylene glycol (PEG) chains, can influence the surface hydration layer to prevent protein adsorption and control blood clotting to a limited extent (Barstad, R. M, et al., Thrombosis and haemostasis 79, 302-305 (1998); Niimi, Y., et al., Anesth. Analg. 89, 573-579 (1999); Chen, S. et al., Polymer 51, 5283-5293 (2010)). However, they are not fully effective and soluble anticoagulants still must be added to the blood.
Bacteria exist in their natural state predominantly as members of biofilms—structured, multicellular communities adherent to surfaces in natural and anthropogenic environments. These communities are composed of many cells embedded within a polymeric organic matrix. Biofilm formation is of concern to industry and healthcare because it causes contamination of plumbing, oil wells, heat exchangers, building ventilation, food storage, medical implants, and other systems. Biofilms threaten human health by triggering an immune response, releasing harmful endotoxins and exotoxins, and clogging indwelling catheters; in fact, biofilms are responsible for nearly 100,000 nosocomial deaths annually in the United States and 80% or more of all microbial infections in humans.
Systemic and topical antimicrobial products have become extensively used to combat biofilm contamination in health care, agriculture, and industrial settings, and increasingly by the general public as well. Commercial products employ a wide variety of active chemical agents, or biocides, often delivered in liquid form and sometimes as vapor. One review of antiseptics and disinfectants identifies 12 classes of liquid agents and 5 common types of vapor-phase sterilants. Regardless of the particular chemistry or mechanism, biocides must be able to reach the target cell to cause damage. At the multicellular level, therefore, the effective biocide must penetrate into the extracellular matrix (ECM)—the slime-like “cement” of biofilm. Biofilms, however, offer their member cells protection from environmental threats. It has been reported that ECM acts as a diffusion barrier and as a charged binding filter for certain antibiotics, and that it complements enzymes and multidrug resistance pumps on cells that remove antimicrobials. The resistance to threats covers a wide range of treatments: biofilms exposed to chlorine bleach for 60 minutes are reported to still have live cells; biofilms in pipes continuously flushed over 7 days with multiple biocides recolonize the pipes, and biofilms have been reported to survive in bottled iodine solution for up to 15 months. Biofilms' resistance to antimicrobials may be related to the extreme nonwettability of their surface as well as resistance to vapor penetration.
Developing biomedical materials that are resistant to biofilm formation before it causes damage or that prevent its robust attachment would significantly reduce the rate of nosocomial infections and the costs associated with treating them. Many negative effects of bacterial colonization stem from the formation of biofilms as protective structures and the associated cooperative behavior of bacterial cells. Persistently bacteria-resistant materials are difficult to achieve by surface chemistry alone. Even if bacteria are unable to attach directly to a material, nonspecific adsorption of proteins or secreted surfactants to the surface eventually masks the underlying chemical functionality with a “conditioning film.” These organic molecules will change the wettability and surface charge of the original surface, and after about 4 hours, a certain degree of uniformity is reached and the composition of the adsorbed material becomes material independent. Materials that rely on leaching impregnated antimicrobials such as silver ion (Ag+) for their function are furthermore limited by the finite reservoir of the active agent. Furthermore, the use of leaching paints containing copper or triorganotin to resist biofouling on ship hulls is increasingly prohibited because of their high environmental toxicity. Some recent research on the effects of nano- or microscale topographical features on bacterial adhesion and subsequent biofilm formation has suggested a possibly more persistent and environmentally sustainable form of controlling bacterial attachment to surfaces, but no evidence yet suggests that this approach can effectively prevent mature biofilm formation or attachment.
There exists a need for an inexpensive, chemically inactive, synthetic slippery surface capable of repelling fluids, withstanding high-impact pressure, and self-healing.